When a person's heart does not function normally due to, for example, a genetic or acquired condition, various treatments may be prescribed to correct or compensate for the condition. For example, pharmaceutical therapy may be prescribed for a patient or an implantable cardiac device may be implanted in the patient to improve the function of the patient's heart.
In conjunction with such therapy it may be desirable to detect conditions in or apply therapy to one or more chambers of the heart. Accordingly, a typical implantable cardiac device may perform one or more functions including sensing signals generated in the heart, pacing the heart to maintain regular contractions and providing defibrillation shocks to the heart.
To facilitate these sensing and therapy operations, an implanted cardiac device may attach to one or more implantable leads that extend from the cardiac device to one or more implant sites located within, on or near the heart. For example, a cardiac device may be implanted in the chest of the patient beneath the subcutaneous fat of the chest wall and above the muscles and bones of the chest. A set of leads may then be routed from the device through a vein and into the patient's heart. Alternatively, the leads from a cardiac device implanted as discussed above or at some other location may be routed to the heart via the pericardial space. In either case, distal portions of the leads are placed at locations within, on, or adjacent the heart such that electrodes on the leads are positioned at desired locations for sensing, pacing, defibrillation or other operations.
In some applications it may be desirable to measure pressure in one or more chambers of the heart or at some other location in a patient's body. For example, cardiac blood pressure readings may be used to detect various cardiac conditions such as congestive heart failure. By measuring cardiac blood pressure, conditions such as these may be detected and in some cases the patient's therapy may be modified to compensate for these conditions. As an example, if cardiac blood pressure is measured over time, the operation of an implanted cardiac device such as a cardioverter defibrillator may be adjusted or medication may be administered, as necessary, based on the conditions diagnosed as a result of the pressure measurements.
To measure cardiac pressure or other pressure in a patient, a pressure sensor may be incorporated into an implantable lead. For example, in some applications a lead may include a pressure sensor along with other components such as electrodes that may be used to sense electrical signals or apply electrical stimulation therapy. In other applications a pressure sensor may be incorporated into a dedicated lead.
Some types of pressure sensors utilize direct mechanical sensing. In a sensor employing a direct mechanical sensing mechanism, a diaphragm or other component of the sensor that is used to measure pressure is in direct mechanical contact with the medium to be sensed. For example, such a sensor may include a diaphragm on an external wall whereby the sensor measures the pressure directly applied to the diaphragm by the medium.
A direct mechanical sensing sensor may be implemented in various ways. For example, a strain gauge-based sensor may include a flexible membrane (a diaphragm) upon which one or more semiconductor stain gauges are mounted. Here, a change in pressure causes deflection of the membrane which, in turn, results in a change in the resistance of the strain gauge which may then be correlated to the change in pressure. Alternatively, a capacitor-based sensor may include a capacitor structure that includes a flexible metal membrane separated via a gap from a free-loading standoff plate. Here, a change in pressure causes deflection of the membrane which, in turn, results in a change in the capacitance of the capacitor which may then be correlated to the change in pressure.
Other types of pressure sensors employ indirect mechanical sensing. For example, a sensor module may incorporate a sensor within a sensor housing where a diaphragm of the sensor is in contact with a fluid, a gel, or some other material that facilitates the transfer of pressure waves. An outer wall of the sensor housing includes a diaphragm that also is in contact with the pressure transfer material. In this case, a change in pressure imparted on the outer diaphragm causes pressure waves to be transferred through the pressure transfer material to the diaphragm of the sensor.
In practice, sensors such as those discussed above may be relatively large (e.g., having a diameter greater than 9 French) due to physical constraints relating to the construction of the sensor. Such constraints may relate to, for example, size limitations of the strain gauge or the capacitor, sensor mounting technologies, or flexible membranes. As a result, it may be undesirable to incorporate such sensors into implantable cardiac leads having relatively small diameters (e.g., on the order of 4-6 French) because the large sensors may make it more difficult to route the lead through small spaces or around sharp corners.
Moreover, these types of sensors may not provide sufficient accuracy to measure certain pressure-related conditions in a patient. For example, a sensor employing direct mechanical sensing may not generate output signals that are correlated in a sufficiently linear manner with changes in pressure. In addition, such a sensor may have unacceptable levels of drift. The above problems may be caused by various factors relating to the design and construction of a sensor including, for example, the thickness of a metal diaphragm (e.g., on the order of 0.001 inches), the interface between a sensor housing and the flexible membrane, distortion caused by manufacturing processes such as welding or brazing, and the long term stability of adhesives or other materials used in the sensor. In addition, due to the design parameters of such a sensor, the sensor may be manufactured using a non-automated process that potentially results in a relatively low manufacturing yield.
Similarly, a sensor employing indirect mechanical pressure sensing may not provide sufficient accuracy to measure certain pressure-related conditions in a patient. For example, in general it is desirable to provide a fluid pressure inside the sensor that is identical to or has a linear relationship with the pressure external to the sensor. In practice, however, the thermal expansion and contraction properties of the pressure coupling material (e.g., fluid or gel) may be significantly different than the corresponding properties of the solid diaphragm and the inner volume holding the pressure coupling material (e.g., contraction or expansion of the space defined by the interior of the sensor housing). Consequently, changes in temperature may result in relatively large changes in the measured pressure. Moreover, the sensor may not linearly track changes in pressure when the sensor is subjected to changes in temperature. In some applications it may not be easy or practical to compensate for these deficiencies.
In view of the above, conventional sensors may not provide sufficiently accurate pressure readings or may not be of a desirable size for implant. Consequently, a need exist for an improved implantable sensor.